The present invention relates to an improvement in matrix heat exchanger construction. These heat exchangers are appropriate where it is desirable to maintain two fluid streams between which heat is to be transferred within separated conduit courses. Heat is conducted from one conduit or tube to the other through a solid medium or matrix surrounding the conduits or tubes. Matrix heat exchangers are to be distinguished from shell and tube, concentric tube and other heat exchanger designs in which process fluids flow through courses separated by a single wall or barrier through which heat is transferred and leaks can result in intermixing of the process fluids.
Matrix heat exchangers are often considered for use in heat-transfer applications involving liquid metal to water or steam. Such applications might include steam generators, steam superheaters or steam reheaters employed in conjunction with liquid-metal-cooled nuclear reactors. Since sodium and sodium-potassium liquid metals are often employed as primary coolants, it is of upmost importance that such reactive metals not be allowed into contact with water or steam in the unlikely event of an accidental leak. A matrix heat exchanger design can be used to minimize the possibility of a liquid metal and water reaction.
A number of limitations have arisen in the design of previous matrix heat exchangers. In some constructions the matrices and tubes have been provided in close, intimate contact such as by casting the matrix material in molten state around an assemblage of tubes or by mechanically bonding e.g. expanding the tubes, into a previously formed matrix. Such constructions may be subject to separation or cracking of the tubes and/or matrix during thermal expansion and contraction produced by high-temperature process cycles. Even very narrow gaps or spaces formed between the tubes and matrix can greatly impair heat transfer. Under same circumstances thermal cycling with resulting contraction and expansion of the tubes may produce a ratchet-like or jacking effect in which tubes slowly work out of the matrix.
In other forms of construction a solder or film is deposited on external surfaces of the tubes prior to assembly. The solder is then made molten or soft to flow into any voids which may exist between the tube and matrix. This type construction depends on the adherence of the solder to the matrix and conduit to prevent gaps. When the solder becomes soft or molten it may not adequately fill existing gaps or it may separate and bead up to produce other gaps with poor conductive coupling between the tubes and matrix. Solder or alloys exhibiting low surface tension and/or inability to wet the tube and matrix materials may be particularly susceptable to such interstitial gap formation.